1. Field of the Invention
The present invention relates to a method and tool for forming precisely curved surfaces, in particular, useful for reflectors of electromagnetic radiation and microwave antennas.
2. Description of the Prior Art
Such reflectors and antennas conventionally are produced in a tedious and costly manner. For example, fabrication of a reflector having a parabolic surface, such as is used in reflector telescope mirrors, requires time-consuming optical polishing of glass or ceramic substrates and subsequent metallizing, and include refined grinding and polishing techniques. Alternate techniques, like centrifugal casting on a rotating turntable, contour turning on a tracer lathe, and plastic injection molding in optical precision molds, have been successfully employed in the past. These manufacturing techniques involve high costs as a result of high labor content and/or expensive tooling. In a like manner, other reflectors are produced by costly electro-forming or precision casting techniques.
Further techniques for shaping articles may be classified among that art relating to stretch and die forming of metal stock, as typified by U.S. Pat. No. 2,961,028, and that to plastic reinforced reflectors and antennas as shown in U.S. Pat. Nos. 3,337,660; 3,374,482; 3,658,971 and 3,897,294. U.S. Pat. No. 2,961,028 preconditions stock by stretching it above its elastic limit to render it more flowable and plastic before being work-hardened as it is formed between matching surfaces of drawing dies. In U.S. Pat. No. 3,337,660 a thin circular diaphragm or membrane, which is clamped around its periphery, is stretched beyond its elastic limit by differential pressure, after which a solidifiable substance, such as an epoxy resin or a polyurethane foam, is poured onto and hardened on the diaphragm. U.S. Pat. No. 3,374,482 molds a foamed-in plastic onto a preformed metallized reflective surface. U.S. Pat. No. 3,658,971 describes a process wherein a foamable composition is injected within a cavity closed by an adhesived strip. Knife edges about the mold peripheries of the cavity sever the adhesived strip upon completion of the expansion of the foamable material, to enable removal of the molded article. U.S. Pat. No. 3,897,294 describes an involved method of forming a parabolic antenna reflector provided with machined slots and holes for receipt of a waveguide feedhorn, and mounting structure for a hyperbolic surface. The parabolic reflector is formed, in part, from a flat copper or copper plastic sheet which is subjected to pressure exerted by a specially prepared paraboloid and mold ring.
These prior art devices are incapable of producing precision reflectors and antennas in that the stretching forces exerted on the materials are anisotropic, i.e., not extending equally in all directions across the surface of the material, which result in astigmatism of the deformed surface. Freeblown shapes also suffer from large aberrations and uncontrolled focal length.